I think that showing the treetops was a way to show Ivy's POV -- that there was a creature, but that she couldn't see it. I don't think a blank screen would have worked. I don't really think that worked, either, but I believe that was his point. Continually keeping it just off screen to reveal it suddenly would have worked better, IMO.

All of this discussion, by the way, doesn't necessarily mean that I liked the movie. Parts of it were good and parts were bad, but I don't think I'd watch it again. Whereas I own Unbreakable and have watched it many times.